Thank you for your post and my apologies in the delayed response.
The dialog units can be set via the 'Dialog Units' field in the Settings Tool (go to Extensions > PlusSpec > Settings). If you change the field value to 'in' then all dialog units will be inches, however, if the field value is 'use model default' then change the units inside the 'Units' section of the SketchUp Model Info dialog (go to Window > Model Info and select Units from the menu on the left). All dialog units should then display in inches and if you change the value (e.g. to 16") then that value will be retained when the dialog is reopened.
Would you be able to please let me know if this works for you?